In the automatic testing of electrical circuits, test probes of various configurations are used, depending upon the type of electrical device under test. A type of test probe commonly referred to as a "switch probe" is used in the automatic electrical continuity testing of electrical equipment such as cable harnesses for automobiles. Switch probes have been used in the past by pressing each probe into contact with a corresponding electrical terminal on the cable harness. Typically, these cable harnesses can have a large number of closely spaced terminals, and a large number of switch probes are mounted to a test head which "cycles" the switch probes in unison to make contact with the terminals with a rapid up-down motion.
A relatively large push out force is applied to each cable harness terminal by the switch probes during each cycle. If the terminal is not seated properly in the cable harness, the terminal is pushed out of the way by the probe, and this prevents the switch from closing. This produces an open circuit in the tester, indicating that the terminal was not seated or mounted properly. On the other hand, if the terminal is mounted properly in the harness, the pressure applied by the switch probe to the terminal during the cycle causes the switch probe plunger to travel far enough to close the switch. This automatically produces a closed circuit in the tester, indicating that the cable harness terminal has been seated or mounted properly.
A typical switch probe is a miniature-sized device which includes an outer barrel, a terminal at one end of the barrel, a movable plunger projecting from the opposite end of the barrel, and a coil spring inside the barrel between the plunger and the terminal. A downward force applied to the plunger against the bias of the spring moves the plunger toward a switch point on the terminal inside the probe. The force applied under test is controlled by the spring constant of the coil spring. If the cable harness is faulty, maximum design force is not reached and the plunger does not close the switch. If the cable harness terminal is wired properly, the maximum design force is reached and the plunger contacts the switch point.
These switch probes are subjected to an extreme amount of abuse because they are constantly cycled at rapid intervals for testing millions of cable harness terminals. Since the terminals under test can be closely spaced apart, they require switch probes of miniature size. Cable harness testing typically requires probes spaced apart by a distance of about one-tenth inch on-center, for example. In one prior art switch probe, the diameter of each probe is about 0.080 inch; another prior art switch probe has a diameter of about 0.125 inch. Cable harness testing also requires a reasonably high push out force, given the size of the probe, the force generally being in the range of 4 to 12 pounds applied over a relatively short distance. Typically, the probes are moved only about 1/10 inch during each cycle.
It is difficult to manufacture coil-type compression springs of the required miniature size for switch probes designed to control the relatively large amount of spring force applied during testing, while also maintaining the necessary reliable operation from cycle-to-cycle over an extremely large number of operating cycles. Probe travel inside the switch probe is particularly critical. If the probe has a slight amount of overtravel or undertravel, for example, the amount of spring force applied to the cable harness terminal changes; and tolerances in the probe spring are difficult to control for a small-sized spring. Fixture tolerances also add to the problem of producing the desired spring force on the cable harness terminal with the conventional spring probe.